The invention concerns an optical sensor for determining angles of rotation.
Such optical systems all work on the principle of the object to be measured moving in a measurement direction relative to a stationary scanning system. Measurements concerning the object are then processed as light and directed onto electrical photoreceivers.
One commonly used principle for operating such optical systems is the transmitted light principle, in which the object, which could be made of glass, transparent plastic, or metal with light-transparent structures etched out, is penetrated by light, preferably parallel light. On the opposite side, the light which has been modulated by the movement of the object is transformed by photoreceivers into electrical signals. This method is relatively simple and economical to carry out.
Another principle used for such optical sensors is optically scanning the object for light reflected by it. Less common, because it is relatively costly and demands more precision parts, is the principle of interferometric scanning of the object. In this case, coherent light interferes on a lattice with a lattice width on the order of the wavelength of the light to produce an extinction or amplification of the light and, thus, a corresponding electrical modulation at the optical receiver. It is known to employ this technique in sensors employing either the transmitted light principle or the reflection principle.
The fundamental problems of all principles are, first, that the distance or spacing between the rotating object and the optical receiver or a static object must be precisely controlled for good modulation and, secondly, the two objects must be accurately centered relative to each other.
In industry, distances of between 100 to 300 μm are customary for optical sensors employing the transmitted light principle. But such relatively large distances are only attainable with modulation structures that are of about the same order of magnitude. For shaft encoders with very high resolution or with very small divisions, the distances must be kept much smaller, which then entails high costs for the mechanical support and the required precision mechanics. The centering of the objects relative to each other is done either optically with appropriate magnification optics or electrically by scanning processed optical signals. In all applications, the rotating object is supported by a separate shaft bearing, which controls both the distance of and the centering relative to the static object. Such shaft encoders are known, for example, from German patent publication DE 100 60 574 A1.